The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine's exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
A technology that has been developed to reduce the levels of NO emissions in lean-burn engines (ex. diesel engines) that burn fuel in excess oxygen includes a selective catalytic reduction (“SCR”) device. The SCR catalyst composition preferably contains a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium which can operate efficiently to reduce NO constituents in the exhaust gas in the presence of a reductant such as ammonia (‘NH3”). Although the use of a catalyst aides in the reduction of activation energy required for the SCR device to operate, the ever increasing efficiency of diesel and other lean burn engines results in cooler exhaust temperatures when moderately operated and following engine start-up. Such cooler operating temperatures delay the operational start-up of the SCR device, which needs to reach a minimum operating temperature to effectively reduce NOx. Typically, an SCR may not reach appropriate operating temperatures until several minutes after the engine is started. Such delay is no longer feasible in view of ever tightening motor vehicle emissions regulations. A primary contributor to slow catalyst light-off, besides the lower exhaust temperatures experienced, is the thermal mass of the engine and the exhaust system that extends between the engine and the SCR device. The thermal mass may include the engine, the engine exhaust manifold, an oxidation catalyst (“OC”) device as well as the exhaust conduit.
A technology that may be used to assist in the control of emitted NOx, during periods when the SCR device is heating to operational temperatures following a cold engine start, or when the SCR device cools below its operating temperature during light load operation, is a NOx adsorber device that traps and holds NOx constituents below temperatures at which the SCR device is operational. The NOx adsorber device is a passive device that includes an adsorbent coating that may comprise a temperature based zeolite NOx absorber compound or other suitable adsorbent compounds that collect and hold NOx constituents at low temperatures. As the exhaust system temperatures rise, the adsorbent compounds release the stored NOx constituents for conversion by the SCR device. A challenge when using passive NOx adsorber devices is to assure that the device is purged of stored NOx constituents when the engine is shut down so that the device will be ready to operate when the engine is restarted.